Difunctional catalyst effective in wax hydroisomerization and process for preparing it

ABSTRACT

Process for the hydroisomerization of n-paraffins in the presence of a difunctional catalyst which comprises: (a) a porous crystalline material isostructural with beta-zeolite selected from boro-silicate (BOR-B) and boro-alumino-silicate (AL-BOR-B) in which the molar SiO2:Al2O3 ratio is higher than 300:1; (b) one or more metal(s) belonging to Group VIIIA, selected from platinum and palladium, in an amount comprised within the range of from 0.05 to 5% by weight.

This application is a division of application Ser. No. 08/975,276 filedon Nov. 20, 1997, now U.S. Pat. No. 5,908,968, which is a Continuationof Ser. No. 08/604,190, filed Feb. 21, 1996, abandoned, which is acontinuation of Ser. No. 08/275,413 filed Jul. 15, 1994, abandoned.

The present invention relates to a difunctional catalyst constituted bya boro-silicate or a boro-alumino-silicate isostructural withbeta-zeolite and one or more metal(s) belonging to Group VIIIA, to itspreparation and to its use in the hydroisomerization of long chainn-paraffins having more than 15 carbon atoms.

The process of wax isomerization to yield base stocks for lubricant oilscharacterized by a Low pour point and a high viscosity index requiresthe use of suitable catalysts.

In fact, waxes, mainly constituted (>80% by weight) by n-paraffinshaving more than 15 carbon atoms, which are solid at room temperature,must be converted into their corresponding branched isomers, havinglower melting point than the linear isomer.

In fact, n-C₁₆ paraffins has a melting point of 19° C., whilst its5-methyl-pentadecane isomer melts at -31° C.

However, an effective hydroisomerization catalyst should minimize thepossible cracking and hydrocracking reactions, which are catalyzed bythe same acidic sites and have, as intermediates, the same carbocationswhich are useful for hydroisomerization. These secondary reactions causethe degradation of the molecule with lighter, less valuable, productsbeing formed, which must be removed from the end product because theyincrease the volatility thereof; this requirement obviously constitutesa burden for the overall process.

For this process, difunctional catalysts have been developed, i.e.,catalysts which display both acidic sites and hydro-dehydrogenationactive sites. The acidity of the catalyst depends on the selected typeof amorphous or crystalline carrier for it, and the function thereofprecisely is the isomerizing activity.

The hydro-dehydrogenating activity is given to the catalyst by thedeposited metal phase, the function of which is also of minimizingcracking.

It was demonstrated (J. F. Le Page, Applied Heterogeneous Catalysis, Ed.Technip, Paris, 1987, 435-466) that, with the hydrogenating activitybeing the same, the most selective catalysts are those in which thecarrier has a controlled acidity, so as to maximize n-paraffinisomerization, relatively to cracking. However, inasmuch as the crackingreactions follow the isomerization reactions, the maximal isomerizationselectivity is obtained at low conversion levels (G. Froment et al.,Ind. Eng. Chem. Prod. Res. Dev., 1981, 20, 654-660). In general, the waxconversion rates are limited to values of 20-40% by weight, in order tominimize the formation of light products caused by the crackingreactions.

The effectiveness of various catalysts can be evaluated on modelcompounds, as n-paraffins, by measuring the selectivity to isomerizationproducts for a given n-paraffin conversion rate.

The use of zeolites functionalized with metals of Group VIII for thehydroisomerization of waxes in order to produce base stocks forlubricant oils has been reported many times. For example, EP-A-440,540claims the use of omega zeolite, EP-A-431,448 of ZSM-5, U.S. Pat. No.4,541,919 claims the use of X, Y zeolites exchanged with alkaline-earthmetals.

U.S. Pat. No. 4,419,220; U.S. Pat. No. 4,518,485; U.S. Pat. No.4,975,177; U.S. Pat. No. 4,554,065; EP-A-464,546; U.S. Pat. No.4,788,378 claim the use of variously modified beta zeolite, as it willbe disclosed hereinunder, plus a hydro-dehydrogenating component.

Zeolites with beta structure can be prepared as alumino-silicates (U.S.Pat. No. 3,308,069); as boro-silicates (Bel. Pat. 877,205; U.S. Pat. No.5,110,570), or as boro-alumino-silicates (EP-A-172,715; U.S. Pat. No.5,110,570).

In the hydrothermal synthesis, the alumino-silicate with beta structurecan be obtained with a maximal molar SiO₂ :Al₂ O₃ ratio of 100:1, asdisclosed in U.S. Pat. No. 4,518,485, column 7, line 12.

Aluminum present in zeolitic structures is known to be responsible forthe formation of acidic sites. From the above patents, such sites resultto be necessary in order to favour the isomerization reaction, butshould a too large number of them be present, they would favourundesired cracking reactions.

Due to this reason, the prior art claims the use of beta zeolites withcontrolled acidity, with said control being performed on the number ofacidic sites, i.e., on the amount of aluminum contained in the catalyst.

Such a parameter can be modified according to two different approaches:

(1) Removal of aluminum by acid extraction and/or heat treatment in thepresence of steam, as disclosed in U.S. Pat. No. 4,419,220; U.S. Pat.No. 4,518,485; U.S. Pat. No. 4,975,177; U.S. Pat. No. 4,554,065. In thatway, molar ratios higher than 100:1, or even as high as 250:1 or 500:1,can be obtained.

(2) Hydrothermal synthesis of boro-alumino-silicate with beta structure.

The partial replacement of aluminum by boron makes it possible betazeolite with a higher molar SiO₂ :Al₂ O₃ ratio to be synthesized. Inorder to obtain materials the acidity of which has been reduced to a lowenough level, i.e., such as to favour isomerization instead of cracking,the need remains however for the material obtained by direct synthesisto be further modified, either by reducing the level of aluminumcontained in it by steaming (EP-A-464,546) or replacing boron by meansof a treatment with SiCl₄ (U.S. Pat. No. 4,788,378).

Summing-up, on the basis of the prior art, beta zeolite results to beactive in the isomerization reaction and, as such, can be used in thepresence of a hydro-dehydrogenating function in the hydroisomerizationof n-paraffins. Its isomerizing activity is given by the presence of theacidic sites generated by the aluminum atoms contained in its framework.The number of such sites has an influence on the selectivity of theisomerization reaction, at the detriment of the undesired crackingraction.

The need for limiting the number of acidic sites present in the materialimplies a considerable burden in terms of preparation steps and catalystcost.

The only declared usefulness of boron introduction claimed in U.S. Pat.No. 4,788,378 and EP-A-464,546 is of making possible the aluminumentered during the step of hydrothermal synthesis to be reduced, withfurther steps for material modifications remaining anyway necessary. Onthe basis of the prior art, no catalytic activity can be anticipated inthe reaction taken into consideration for boron, the removal of which,to the contrary, appears to be useful (U.S. Pat. No. 4,788,378).

The present invention relates to a process for preparing a catalystwhich is effective in the hydroisomerization of waxes, which catalystdoes not display the drawbacks which affect the prior art, i.e., it doesnot require any further treatments, such as aluminum removal or boronreplacement.

On the contrary, the present Applicant unexpectedly found that when abeta zeolite is used in which all aluminum was replaced by boron duringthe hydrothermal synthesis (BOR-B, according to Bel. Pat. 877,205), plusone effective component in hydrodehydrogenation, high isomerizationselectivities are obtained, with the activity of the material towardscracking resulting to be very low.

Traces of aluminum can be entered during the hydrothermal synthesis witha good selectivity material was still obtained, provided that the molarSiO₂ :Al₂ O₃ ratio is higher than 300:1. Such materials obtained bydirect synthesis do not require any subsequent treatment aiming atremoving aluminum or anyway reducing the number of acidic sites, withthe preparation process being considerably simplified.

The end synthesized products display alpha-test values comprised withinthe range of from 1 to 5.

In accordance therewith, the present invention relates to a process forthe hydroisomerization of n-paraffins having more than 15 carbon atoms,characterized in that n-paraffin, or the mixture of n-paraffins, isbrought into contact, under hydroisomerization conditions, with adifunctional catalyst which comprises:

(a) a porous crystalline material isostructural with beta-zeoliteselected from boro-silicate (BOR-B) and boro-alumino-silicate (AL-BOR-B)in which the molar SiO₂ :Al₂ O₃ ratio is higher than 300:1, preferablyhigher than 500:1;

(b) one or more metal(s) belonging to Group VIIIA, preferably selectedfrom platinum and palladium, in an amount comprised within the range offrom 0.05 to 5% by weight, preferably from 0.1 to 1% by weight.

A further object of the present invention is a process for preparing thecatalyst useful in the hydroisomerization process disclosed above.

Such a process consists of two steps, the first of which relates to thepreparation of BOR-B or AL-OR-B isostructural with beta zeolite, thesecond one relates to the impregnation of said porous crystallinematerial with a metal belonging to Group VIIIA.

In accordance therewith, the process for preparing the catalyst to beused in the hydroisomerization process comprises:

(1) a first step, of preparation of the porous crystalline materialwhich is isostructural with beta zeolite, selected from boro-silicateand boro-alumino-silicate;

(2) a second step, in which the zeolite obtained from the (a) step istreated with a metal from Group VIIIA.

The first step, as disclosed in U.S. Pat. No. 5,110,570 consists inpreparing a mixture containing a silica source, a boron source, analkali metal hydroxide (MEOH), a tetra alkyl ammonium salt (R⁺),distilled water and optionally, an aluminum source, so that the molarcomposition of the mixture, expressed as oxides, is the following:

* SiO₂ /B₂ O₃ >1;

* R/SiO₂ =0.1-1.0;

* ME/SiO₂ =0.01-1.0;

* H₂ O/SiO₂ =5-80;

and, in the case of boro-alumino-silicates:

* SiO₂ /Al₂ O₃ >300, preferably>500.

To such a mixture, an aliquot of from 1 to 60% of crystallization seedscan possibly be added, which seeds have the same composition and havebeen previously submitted to partial crystallization under hydrothermalconditions under the autogenous pressure of the reaction mixture, at atemperature comprised within the range of from 90 to 160° C., during aperiod of at least one day.

The resulting mixture is heated in an autoclave, under hydrothermalconditions, under autogenous pressure, at a temperature comprised withinthe range of from 90 to 160° C.

The crystallization time is comprised within the range of from 1 to 7days, with it being shorter in the presence of seeds.

At the end of the process, the reaction mixture is discharged from theautoclave, the crystalline material is recovered by filtration, iswashed with distilled water and is dried at 120° C. for some hours. Theresulting product is submitted to ion exchange in order to be turnedinto its acidic form, according to the processes known from the priorart.

The silica source which can be used in the process according to thepresent invention can be selected from colloidal silica, silica gel,sodium silicate, and so forth, preferably colloidal silica; the boronsource is selected from boric acid, alkaline borates, trialkyl borates,and so forth, with boric acid being preferred. As the organic template,a tetra alkyl ammonium salt, preferably tetra ethyl ammonium hydroxide,is used.

The zeolite in acidic form obtained in that way is then submitted (step2) to a process which makes it possible it to be at least partiallycoated with a metal from Group VIIIA, preferably palladium or platinum.

Said second step can be carried out by impregnation from an aqueousmedium, or by ion exchange, preferably by impregnation.

According to the impregnating technique, the zeolite obtained from thefirst step is wetted with an aqueous solution of a metal compound, e.g.,chloroplatinic acid or Pd(NH₃)₄ (NO₃)₂, by operating either at roomtemperature, or at close-to-room temperatures.

After the aqueous impregnation, the solid material is dried, preferablyin air, at room temperature, or at close-to-room temperatures, and issubmitted to thermal treatment under an oxidizing atmosphere, preferablyair. Suitable temperatures for for this heat treatment are comprisedwithin the range of from 200 to 600° C. The conditions are so adjustedas to deposit on the particles (prepared as disclosed in step 1), ametal amount which is comprised within the range of from 0.05 to 5%,preferably of from 0.1 to 2%.

According to the ion exchange technique, the zeolite obtained from thestep 1 is suspended in an aqueous solution of a noble metal complex orsalt, e.g., an aqueous solution of Pt(NH₃)₄ (OH)₂, Pt(NH₃)₄ Cl₂,Pd(NH₃)₄ (NO₃)₂, operating either at room temperature or toclose-to-room temperatures. After the ion exchange the resulting solidmaterial is separated, washed with water, dried and finally submitted tothermal treatment under an inert and/or oxidizing atmosphere. Heattreatment temperatures comprised within the range of from 200 to 600° C.have been found to be useful for the intended purpose. The conditionsare so adjusted as to deposit on treated zeolite particles a metalamount which is comprised within the range of from 0.05 to 5% by weight,preferably of from 0.1 to 2%.

The catalysts according to the present invention can be used as such, orin combination with suitable inert, solid material acting as binders.

Oxides of the type of silica, alumina, titania, magnesia and zirconia,which may be taken either individually or combined with each other,result to be suitable. The catalyst and the binder can be mixed in amutual ratio, by weight, comprised within the range of from 30:70 to90:10, and preferably of from 50:50 to 70:30. Both components can becompacted giving them a desired end shape, e.g., as extrudates, orpellets.

The catalysts obtained from the process according to the presentinvention can be activated by drying and/or reduction, preferably bydrying and subsequent reduction. The drying is carried out under aninert atmosphere at temperature comprised within the range of from 100to 400° C., and the reduction is accomplished by submitting the sampleto thermal treatment under a reducing atmosphere, at a temperaturecomprised within the range of from 150 to 500° C.

The catalyst prepared according to the above techniques, is active inthe hydroisomerization process, which can be carried out eithercontinuously or batchwise.

The hydroisomerization is suitably carried out in the presence of H₂, ata temperature comprised within the range of from 200 to 540° C.,preferably of from 250 to 450° C., and under a pressure of fromatmospheric pressure up to 25,000 kPa, and preferably of from 4,000 to10,000 kPa.

The effective catalyst amount, as expressed as weight percentage basedon n-paraffins or n-paraffin mixtures to be hydroisomerized, isgenerally comprised within the range of from 0.5 to 30% by weight,preferably of from 1 to 15% by weight.

The following experimental examples are reported in order to betterillustrate the present invention.

EXAMPLE 1 (Catalyst 1)

This boro-silicate is disclosed in BE-877,205. 3.0 grams of NaOH and 6.4grams of boric acid are dissolved in 28.1 g of an aqueous solution at40% of tetra ethyl ammonium hydroxide. A clear solution is obtainedwhich is diluted with 30.0 g of distilled water and is added to 51.0 gof Ludox AS silica, containing 30% by weight of silica.

The resulting suspension, having a pH value of 12.2, is kept stirred atroom temperature for 4 hours and is then charged to an autoclave inorder to crystallize under static conditions, under its autogenouspressure, at 150° C. for 7 days.

At the end of this time period, the resulting product is discharged,washed and dried.

The material was characterized by X ray analysis, with the structure ofa pure BOR-B being evidenced.

The product is calcined at 550° C. for 5 hours, is exchanged into itsacidic form by treatment with a solution of ammonium acetate andsubsequent calcination under the above indicated conditions.

The resulting material displays a molar ratio of SiO₂ :B₂ O₃ =45 and,when it was tested in the cracking reaction with n-hexane, it yielded analpha value equal to 1.

The Pt metal phase is deposited onto the beta zeolite by aqueousimpregnation.

In particular, on 10 g of beta zeolite, prepared as disclosed above,charged to a crystallizer, 12.6 mL of an aqueous solution containing H₂PtCl₆ (0.45% by weight/volume) and (0.6M) HCl is added dropwise, mixingaccurately. The reactants are allowed to stay into contact for 16 hours,water is then evaporated off by heating at 60° C. for 1 hour, and thesample is subsequently dried for 2 hour at 150° C., still in air. Thecalcination is carried out at 500° C. for 3 hours under a flowing airstream, with the muffle being heated from 23 to 500° C. during 90minutes.

EXAMPLE 2 (Catalyst 2)

A suspension of seeds for the synthesis of Al-BOR-B is prepared byoperating according to as disclosed in U.S. Pat. No. 5,110,570, asfollows:

Four grams of NaOH and 8 grams of boric acid are dissolved in 30 g ofdistilled water. To this solution, 35 g of tetra ethyl ammoniumhydroxide at 40% in water and 0.2 g of Al(NO₃)₃.9H₂ O previouslydissolved in 10 g of water, are added. The end solution obtained in thatway is added to 64 g of Ludox AS silica, at 30% by weight.

A mixture "A" is obtained which is allowed to stay at room temperatureduring about 4 hours, and is then charged to an autoclave and is causedto crystallize at 150° C. for 5 days, under static conditions and underits autogenous pressure.

Twenty-six grams of so prepared seed suspension is added to 155 g of amixture having the same composition as of mixture A.

After a 3-day crystallization at 150° C., with stirring, under theautogenous pressure, an aluminum-containing BOR-B is obtained. Afterbeing exchanged into its acidic form the product displays an alpha value=4. Its composition is as follows:

* SiO₂ /Al₂ O₃ =698;

* SiO₂ /B₂ O₃ =41.

The so prepared boro-alumino-silicate is submitted to the platinumimpregnation step as disclosed in above Example 1.

COMPARISON EXAMPLE 3 (Catalyst 3)

A reactant mixture "A" is prepared by dissolving 0.8 g of NaOH, 0.4 g ofNaAlO₂ and 3.7 g of H₃ BO₃ in 65.5 g of tetra ethyl ammonium hydroxideat 14% by weight. To the resulting clear solution, 31.2 g of Ludox HSsilica at 40% by weight is added. The resulting "A" mixture is chargedto an autoclave and is allowed to crystallize for 2 days at 150° C.under static conditions and under its autogenous pressure. In that way,a milky seed suspension is obtained.

Twenty-four grams of this milky seed suspension is added to 130 g of amixture having the same composition as of "A" mixture. The resultingsuspension is allowed to crystallize at 150° C., under its autogenouspressure, under static conditions, for 2 days.

After being submitted to exchange into its acidic form, the product hasthe following molar composition:

* SiO₂ /B₂ O₃ =43;

* SiO₂ /Al₂ O₃ =90.

The resulting AL-BOR-B is impregnated with platinum, by operatingaccording to the same process as disclosed in above Example 1.

COMPARISON EXAMPLE 4 (Catalyst 4)

A beta-zeolite is prepared under similar conditions to as disclosed inU.S. Pat. No. 3,308,069.

An amount of 59.8 g of tetraethylammonium hydroxide (TEA-OH) at 40% byweight/weight in aqueous solution and 1.9 g of sodium aluminate areadded to 58.4 g of demineralized water. The resulting mixture is heatedup to about 80° C. and is kept with stirring, until NaAlO₂ is completelydissolved. The obtained solution is added to 48.7 g of Ludox NScolloidal silica at 40% by weight, such as to yield a molar ratio ofSiO₂ :Al₂ O₃ =28.

The resulting homogeneous suspension having pH=14 is charged to astainless steel autoclave and is allowed to crystallize underhydrothermal conditions, in an oven at 150° C. during 10 days, understatic conditions and under its autogenous pressure. The crystallizedproduct, which results to be a pure beta-zeolite, is filtered off, iswashed, is dried at 120° C. during 1 hour, is calcined at 550° C. during5 hours and is exchanged into its acidic form by exchange with ammoniumacetate and subsequent calcination under the above indicated conditions.

The Pt metal phase is deposited on beta-zeolite by aqueous impregnation,as disclosed in Example 1.

EXAMPLE 5

The catalyst from Example 1 was tested in the reaction of n-C₁₆ paraffinhydroisomerization, inside a microautoclave under the followingconditions.

The microautoclave is constituted by a steel body and a lid providedwith a plurality of valves for autoclave pressurizing, venting andpossible recovery of gas products, and with a safety (pressure relief)disk. The stirring system consists of a thin inner metal rod.

The reactor is charged with 8 g of C₁₆ paraffin and 0.25 g of catalyst;the system is pressurized, when cold, at 5 MPa with H₂, and is thenheated up to 360° C.

Zero time is assumed to be that time point at which the temperatureinside the reactor reaches the desired value. When 120 minutes haveelapsed, the reactor is cooled and depressurized, and the reactionmixture is then recovered.

The analysis of the product in order to determine the conversion rateand the product distribution is directly carried out on the resultingmixture by gas-chromatography (HP-1 crosslinked methyl silicone gumcolumn, atomic emission detector).

In Table 1, the values of conversion and selectivity are reported, whichare computed as follows: ##EQU1## in which "iso-C₁₆ " is the mixture ofisomers with 16 carbon atoms.

EXAMPLE 6

A catalyst according to Example 1 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 350° C. The values ofconversion and selectivity are reported in Table 1.

EXAMPLE 7

A catalyst according to Example 1 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 340° C. The values ofconversion and selectivity are reported in Table 1.

EXAMPLE 8

A catalyst according to Example 2 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5.The values of conversion and selectivity are reported in Table 1.

EXAMPLE 9

A catalyst according to Example 2 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for the reaction time, which is reduced down to 60 minutes. Thevalues of conversion and selectivity are reported in Table 1.

EXAMPLE 10

A catalyst according to Example 2 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 350° C. The values ofconversion and selectivity are reported in Table 1.

EXAMPLE 11

A catalyst according to Example 2 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 340° C. The values ofconversion and selectivity are reported in Table 1.

EXAMPLE 12

A catalyst according to Example 2 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 300° C. The values ofconversion and selectivity are reported in Table 1.

COMPARISON EXAMPLE 13

A catalyst according to Example 3 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 6.The values of conversion and selectivity are reported in Table 1.

COMPARISON EXAMPLE 14

A catalyst according to Example 4 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 6.The values of conversion and selectivity are reported in Table 1.

COMPARISON EXAMPLE 15

A catalyst according to Example 4 is tested in the hydroisomerization ofn-C₁₆ paraffin. The conditions are kept equal to those as of Example 5,but for temperature, which is decreased down to 300° C. The values ofconversion and selectivity are reported in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Temperature                                                                             Time t,       Selectivity                              Example Catalyst T, °C. minutes Conversion to iso-C.sub.16           ______________________________________                                                                               %                                      5      1       360       120   65.1    88.3                                     6 1 350 120 48.7 95.9                                                         7 1 340 120 35.6 95.7                                                         8 2 360 120 98.6 70.8                                                         9 2 360  60 52.4 93.1                                                         10 2 350 120 89.0 76.0                                                        11 2 340 120 75.4 89.3                                                        12 2 300 120 17.5 98.9                                                        13 3 350 120 99.6 26.8                                                        14 4 350 120 99.5 21.2                                                        15 4 300 120 89.8 26.1                                                      ______________________________________                                         Reaction conditions:                                                          P.sub.H2 = 5 MPa;                                                             nC.sub.16 :catalyst = 8:0.25.                                            

From Example 5, one may see that the catalyst based on BOR-B and Ptdisplays extremely good values of selectivity to C₁₆ isomers at a higherconversion level than those which are usually taken into considerationin technical papers.

From Examples 5, 6 and 7, one may see that the temperature decreasecauses a decrease in conversion and an increase in selectivity to C₁₆isomers of up to 96%.

From Examples 5 and 8, one may see that the introduction of small Alamounts inside the structure causes, with the other experimentalconditions being the same, an increase in conversion rate, with aconsequent decrease in selectivity to C₁₆ isomers down to 70%.

From Examples 8 and 9, one may see that the decrease in reaction timehalves the conversion rate, with the selectivity to useful productsbeing increased up to 93%.

From Examples 8, 10, 11 and 12, one may see that at too low reactiontemperatures (300° C.), the conversion rate decreases down to lowervalues that 20%, whereas at intermediate temperatures (340° C.), a goodratio of conversion values (75%) to the values of selectivity to C₁₆isomers (90%) is obtained.

From Examples 6, 10, 13 and 14, one may see that zeolites with betastructure, or BOR-B with too high Al contents and not submitted toaluminum removal treatments display, with the other experimentalconditions being the same, very low selectivity values, in the presenceof nearly quantitative conversion levels.

Even if the reaction temperature is decreased down to 300° C. (Example15), the values of conversion and selectivity to useful products are notsubstantially modified.

We claim:
 1. Difunctional catalyst which comprises:(a) a porouscrystalline material isostructural with beta-zeolite selected fromboro-silicate (BOR-B) and boro-alumino-silicate (AL-BOR-B) in which themolar SiO₂ Al₂ O₃ ratio is equal to or higher than 698:1; (b) one ormore metal(s) belonging to Group VIIIA, in an amount within the range offrom 0.05 to 5% by weight, wherein when the beta-zeolite is theboro-silicate (BOR-B), the group VIlIA metal is platinum. 2.Difunctional catalyst according to claim 1, wherein the one or moremetal(s) belonging to Group VIIIA, is present in an amount within therange of from 0.1 to 2% by weight.
 3. Process for preparing the catalystaccording to claim 1, which comprises:(1) preparing the porouscrystalline material which is isostructural with beta zeolite, selectedfrom boro-silicate (BOR-B) and boro-alumino-silicate (AL-BOR-B) in whichthe molar SiO₂ :Al₂ O₃ ratio is equal to or higher than 698:1; (2)treating the zeolite obtained from (1) with a metal from Group VIIIA,with a technique selected from the group consisting of ion exchange andimpregnation.
 4. Process according to claim 3, characterized in thatsaid (b) step is carried out according to the impregnation technique.